Wide-range linear output photo sensor circuit

ABSTRACT

A system and method for acquisition and conditioning of a signal received from a light sensor generating an input signal in proportion to a sensed amount of light, the system utilizing an operational amplifier with first and second feedback loops to render the output signal of a control circuit, part of the second feedback loop, in proportional relationship to the input signal across the range of output signals from the light sensor.

FIELD OF THE INVENTION

The present invention relates to photo sensing circuits and to themeasurement of light using photosensing circuits.

BACKGROUND OF THE INVENTION

The main functional task of a sensor is to produce an output signalrepresentative of the detected amount or level of a measured physicalphenomenon, of the type for which the sensor was pre-designed to detect.The produced output signal is then used by a wide variety of devices fordifferent control or measurement purposes

Sensors will often generate output signals that have relatively lowsignal strength or a high noise component. Accordingly, it is known toprovide such sensors with electronic circuitry for signal conditioningof the output signal so that the conditioned output signal can be moreeasily or more accurately sensed, interpreted or otherwise used by acontrol system. Such conditioning can involve but is not limited toprocesses such as amplification and/or filtering. For example, onesimple and well-known solution for conditioning the output signal of aphotodiode sensor is to use a linear trans-impedance operationalamplifier. One example of such a linear trans-impedance operationalamplifier is a Burr-Brown CMOS Rail-to-Rail IO Operational AmplifierOPA2357 sold by Texas Instruments, Dallas, Tex., USA.

However, in some situations, the use of such linear amplifiers may notbe appropriate. Wide range sensors that theoretically generate a linearoutput signal over several decades may not be particularly compatiblewith such linear amplifiers. For example, photodiode sensors typicallygenerate a response to light that is proportional to the amount of lightreceived. Accordingly, when such sensors are used to detect daylight theamount of input varies widely from darkness to full daylight, and thesensor itself can detect and produce an output signal that is indicativeof the sensed amount of light over a wide dynamic range. However, linearamplification is problematic because a tradeoff is typically made insuch linear amplification circuits between the extent of theamplification (resolution) and operating signal range of the linearamplifier. That is, a linear amplifier that provides an appropriatelevel of resolution when the sensor output signal is configured toprovide relatively low levels of output will typically be driven toprovide an oversaturated response beyond which the linear amplifierprovides no further resolution. Conversely, an amplifier that providesadequate resolution at the higher end of the sensor output willtypically provide inadequate resolution for lower level output signals.

To solve this problem, logarithmic amplifiers can be used. One exampleof such a logarithmic amplifier is a Burr-Brown Precision Logarithmicand Log Ratio Amplifier, LOG104 also sold by Texas Instruments. As isnoted in Texas Instruments' publication SBOS243C, published in May 2002,and revised in April 2005: “The LOG104 is a versatile integrated circuitthat computes the logarithm or log ratio of an input current relative toa reference current. The LOG 104 is operable over a wide dynamic rangeof input signals. In log ratio applications, a signal current can comefrom a photodiode, and a reference current from a resistor in serieswith a precision external reference.” Unfortunately, this solution iscomplicated and requires non-linear conversion of the output signalwhich provides a predetermined relationship which may not match theperformance of a particular sensor.

Accordingly, what is needed in the art are a method and circuitry thatenable wide dynamic range sensing having a linear output throughout theeffective range of a sensor.

SUMMARY OF THE INVENTION

In a first aspect of the invention a sensing and conditioning system areprovided. The sensing and conditioning system comprises: an analog lightsensor capable of converting light incident on a sensing surface into ananalog light sensor current signal indicative of a range of, an amountof, an intensity of, or an exposure of the sensing surface to light; anoperational amplifier having an inverse input connected to the analoglight sensor, and a non-inverse input being connected to a ground, theamplifier providing an amplifier output signal at an amplifier output;an amplifier feedback loop having a resistor connected between theamplifier output and its inverse input to provide a feedback currentsignal at the amplifier inverse input; a comparator having a firstcomparator input connected to the output of the operational amplifierand a second comparator input connected to a reference circuit providinga reference signal, so that the comparator provides an output signal ata comparator output that reflects a comparison between the amplifieroutput signal and the reference signal; a control circuit having acontrol circuit input connected to the output of the comparator withsaid control circuit receiving the output signal from the comparatorand, in response thereto, generating a control circuit output signalthat has at least an integration component; an offset source circuithaving an offset input connected to the control circuit output and anoffset output connected to the inverse input of the operationalamplifier with the offset source circuit being adapted to generate at anoffset current signal at the offset output based upon the controlcircuit output signal, the offset current signal tending to minimize thedifference between the light sensor current signal and the feedbacksignal so that the amplifier output signal will be controlled to beclose enough to the reference signal to cause the control circuit outputsignal to be proportional to the light sensor signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block-diagram describing an embodiment of the widerange photo sensor acquisition and conditioning system;

FIG. 2 is a digital version of a preferred embodiment of the wide rangephoto sensor acquisition and conditioning system;

FIG. 3 is another digital version of a preferred embodiment of thewide-range photo sensor acquisition and conditioning system;

FIG. 4 is another digital version, employing a configurable controller,of a preferred embodiment of the wide-range photo sensor acquisition andconditioning system; and

FIG. 5 is a block diagram of a configurable controller useful for thedigital version of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a general block-diagram showing one embodiment of a sensingand conditioning system 11. In this embodiment, system 11 has an analoglight sensor 13 comprising, a photodiode 15. In other embodiments, lightsensor 13 can comprise, for example, a transducer that is capable ofconverting light incident on a sensing surface into an analog outputsensor signal indicative of a range or an amount, intensity, or anexposure of the sensing surface to light stimulus. In the embodiment ofFIG. 1, photodiode 15 is shown controlling a sensor current signal i(d)in proportion to an amount of received light. In other embodiments,light sensor 13 can comprise, for example, and without limitation, aphotoconductive cell or a photovoltaic cell.

As is also shown in FIG. 1, an operational amplifier 21 is providedhaving an inverse input 23, a non-inverse input 25 and an amplifieroutput 27. Inverse input 23 is connected to light sensor 13, whilenon-inverse input 25 is shown connected to a ground 29. A firstamplifier feedback loop 31 is provided having a feedback resistor 33with a resistance Rf connected between amplifier output 27 and inverseinput 23. In operation, first amplifier feedback loop 31 provides acurrent-to-voltage converter that generates a feedback current signali(f) that works to help convert the sensor current signal i(d) into anamplifier output signal V(u).

A second amplifier feedback loop 32 is shown connected between amplifieroutput 27 and inverse input 23. Second amplifier feedback loop 32comprises a comparator 41, a reference signal source circuit 47, acontrol circuit 51 and an offset source circuit 61. Second amplifierfeedback loop 32 provides an offset current signal i(o) to inverse input23 that has at least an integration (I) component with respect to theamplifier output signal V(u). Examples of such systems that can providean integration component with respect to a signal, such as offsetcurrent signal i(o), are known to those of skill in the art, someexamples of which are provided in “Theory and Problems of Feedback andControl Systems”, Second Edition, page 22, written by Joseph J.Distefano, Allen R. Stubberud, and Ivan J. Williams, publisherMcGRAW-HILL).

Specifically, as shown, comparator 41 is connected at a first comparatorinput 43 to amplifier output 27 and is also connected at a secondcomparator input 45 to reference signal source circuit 47. Referencesignal source circuit 47 provides a reference signal V(ref) tocomparator 41. Comparator 41 provides a comparator output signal V(out)at a comparator output 49. Comparator output signal V(out) reflects acomparison between the amplifier output signal V(u) and reference signalV(ref).

Control circuit 51 has a control circuit input 53 connected tocomparator output 49. In this way, control circuit 51 receives thecomparator output signal V(out) and, in response thereto, generates acontrol circuit output signal V(r) at a control circuit output 55.Control circuit output signal V(r) is based upon the output signalreceived from comparator 41 and is adapted to generate the controlcircuit output signal V(r) to have at least integration (I) componentwith respect to amplifier output signal V(u).

Control circuit 51 can take any of a variety of forms. In one exampleembodiment, control circuit 51 can comprise an integrator circuit havingan operational amplifier which performs the mathematical operation ofintegration such as the integrator circuit shown on NationalSemiconductor Application Note 20, entitled “An Applications Guide forOp Amps”, published February 1969 by National Semiconductor Corporation,Santa Clara, Calif., USA.

In this embodiment, the output of control circuit 51 is connected to theinput of offset source circuit 61 which, in this embodiment, comprises acontrolled current source that provides an offset current signal i(o)that flows to inverse input 23 of operational amplifier 21. Offsetsource circuit 61 can be implemented, for example, as a resistor (notshown) which determines the offset current signal i(o) as a valueproportional to the voltage applied to such a resistor.

Accordingly, in this embodiment, sensing and conditioning system 11operates as follows: after power-up, operational amplifier 21 convertsthe difference of sensor current signal i(d) and the offset currentsignal i(o) into an amplifier output signal V(u).

The expression for V(u) may be written as:

V(u)=i(f)×Rf=[i(d)−i(o)]×Rf  (1)

The input current of operational amplifier 21 at inverse input 23 may beneglected due to a very high input resistance of operational amplifier21.

Amplifier output signal V(u) is compared by comparator 41 with areference signal V(ref) which is typically a pre-set reference voltagesupplied by reference signal source circuit 47. The reference signalV(ref) can comprise, for example, a voltage signal having any valuewithin the amplifier supply voltage V(cc) of amplifier output signalV(u), for example V(ref)=V(cc)/2. Comparator 41 generates a comparatoroutput signal V(out) that forces control circuit 51 to generate anoutput signal V(r). Offset source circuit 61 receives control circuitoutput signal V(r) and generates an output current signal that tends toreduce or minimize the difference between sensor current signal i(d) andoffset current signal i(o).

In other words, amplifier output signal V(u) will be controlled to be asclose as possible to the reference value V(ref) supplied by referencesignal source circuit 47 such that the output signal V (r) of controlcircuit 51 will be proportional to sensor current signal i(d).

To perform the compensation of an increasing sensor current signal i(d),control circuit 51 is adapted to generate a control circuit outputsignal V(r) that causes an increase in the offset current signal i(o)thus in turn generating a larger control circuit output signal V(r). Thecontrol circuit output signal V(r) provides a linear output signalacross the range of possible output signals provided by light sensor 13,due to the linear relationship between the measured light intensity andsensor current signal i(d).

FIG. 2 shows another embodiment sensing and conditioning system 11wherein various portions of sensing and conditioning system 11 areexecuted using digital techniques. As shown in FIG. 2, in thisembodiment, comparator 41, reference signal source circuit 47, andcontrol circuit 51 comprise digital circuits that perform generally asdescribed above, using digital circuitry and signals in lieu of analogcircuitry and signals. To facilitate this, an analog-to-digitalconversion circuit (ADC) 71 and a digital-to-analog conversion circuit(DAC) 73 are added. Specifically, as shown in FIG. 2, amplifier outputsignal V(u) is converted by analog-to-digital conversion circuit (ADC)71 into a digital output signal V(ud) representing the voltage level ofthe output signal and the digital output signal V(ud) is provided to aninput of comparator 41. Similarly, as illustrated in FIG. 2, referencesignal source circuit 47 comprises a source of a digital referencesignal V(refd) which can include by way of example and not by way oflimitation, a digital memory, a processor, a latch or any other suchcircuit that can store and provide digital data comprising digitalreference signal (Vrefd) for use by the digital embodiment of comparator41 illustrated in FIG. 2.

Comparator 41 compares the digital output signal V(ud) and the digitalreference signal V(refd) and generates a digital comparator outputsignal V(outd). Similarly, control circuit 51 receives the digitalcomparator output signal V(outd) and generates a digital control circuitoutput signal V(rd). The digital control circuit output signal V(rd) isreceived by digital-to-analog conversion circuit 73 and converted intoan analog output signal V(r) that is then provided to offset sourcecircuit 61. In the embodiment illustrated in FIG. 2, this digitalcontrol circuit output signal V(rd) is provided in digital form as anoutput signal. Alternatively, where desired, the analog output signalV(r) can be provided in analog form as an output signal.

FIG. 3 shows yet another embodiment of a sensing and conditioning system11. This embodiment incorporates features of the embodiment of FIG. 2,having a control circuit 51 comprising a data latch circuit 81 and aramp generator circuit 83. In this embodiment, ramp generator circuit 83which can comprise, for example, and without limitation a countercircuit, generates a digital ramp output signal V(rampd) at its output85, that is provided to both digital-to-analog converter 73 and to datalatch 81. Digital-to-analog converter 73 converts the digital rampoutput signal V(rampd) to an analog signal controlling offset sourcecircuit 61, thus creating an offset current signal i(o) which isperiodically changing from a predetermined minimum to a maximum offsetcurrent value. This causes the amplifier output signal V(u) to changeaccording to equation (1). At the moment when V(u) will be equal to apredetermined value—for example, to half of the effective range of theamplifier output signal V(u)—set by the digital reference signal V(refd)from reference signal source 47. Comparator 41 then generates theequality signal to latch the corresponding ramp generator 83 outputsignal V(rampd). This latched data, representing the output V(rd) ofdata latch 81, will be proportional to the measured light level atsensor 13. As can be seen from this description the output V(rd) latcheddata will change once per ramp generator 83 period. As known in the art,the ramp frequency should be fast enough to cope with the lightvariations at sensor 13 and slow enough to allow a good resolution at awide measurement range.

FIGS. 4 and 5 show an embodiment similar to the embodiment of FIG. 3wherein functions of digital control circuit 51 are performed by modulesof a configurable control system, as described, for example, in WO2005/029207 entitled CONFIGURABLE CONTROLLER filed Jul. 12, 2004 in thename of Burkatovsky and U.S. Ser. No. 11/472,142 entitled AN ADAPTIVEINPUT-CELL CIRCUITRY USEFUL IN CONFIGURABLE ELECTRONIC CONTROLLERS,filed Jun. 21, 2006 in the name of Burkatovsky. The block diagram of theconfigurable control system 600, as shown originally in FIG. 3 of U.S.Ser. No. 11/472,142, is copied here to FIG. 5.

Specifically, FIG. 5 is a general block diagram of one embodiment of aconfigurable control system 600 that uses a plurality of adaptiveinput-cells 500. Configurable control system 600 is substantiallysimilar to the controller illustrated in FIG. 3 of WO 2005/029207,except for the adaptive input-cell 500, which replaces the originalbasic input-cell 240. Configurable control system 600 comprises asynchronizing signal generator 250, configurable digital unit 200, suchas a FPGA or CPLD, which comprises at least synchronization controlmodule 270, control logic module 370, a number of signal acquisitionmodules 260, configured to accept signals coming from input pins 210 ofconfigurable control system 600 through adaptive input-cells 500.Adaptive input-cells 500 can be identical or can vary.

Configurable output control logic modules 280 can be configured toprovide control of the loads connected to the output pins 380 ofconfigurable control system 600, through high-side output drivers 350and/or low-side output drivers 360. In this embodiment, such high-sideoutput drivers 350 each have a high-side switch control 310 andhigh-side switch 320, while such low-side output drivers 360 each have alow-side switch control 330 and low-side switch 340.

The synchronization control module 270 of configurable control system600 is configured to generate basic time-dependent signals, in order tosynchronize the work of the adaptive input-cells 540 and signalacquisition modules 260. Such synchronization is needed for conversionof input signal values to time-based parameter, (e.g. pulse width,delay, duty cycle, frequency, etc.) by adaptive input-cells 540, andthen for converting these time-based parameters to digital form by meansof configured signal acquisition modules 260. One possibleimplementation of synchronization control module 270 may bc, forexample, a counter, which counts incoming pulses with constant intervalbetween them. The sequence of such pulses can be obtained from thesystem clock, for example. The output reference data 290 ofsynchronization control module 270 is connected to each of the signalacquisition modules 260 and also to synchronizing signal generator 250as a sync data 275. Synchronizing signal generator 250 is implemented,for example, as a digital to analogue converter.

Synchronization control module 270 is connected to each of the signalacquisition modules 260 and also to synchronizing signal generator 250as a sync data 275. While synchronization control module 270 is running,the value of the sync data 275, which is equal to output reference data290, is periodically changed from 0 to its maximum value, which causes asaw-teeth shape synchronization voltage Vsync on the synchronizationinput 255 of synchronizing signal generator 250. This voltage istransferred to the second input of adaptive input-cells 540. First input522 of each adaptive input-cell 540 is connected to a correspondingsignal input pin 210 of configurable control system 600 respectively.The comparator output signal 220 of adaptive input-cell 540 is connectedto the input of corresponding signal acquisition module 260. Theimplementation of configurable signal acquisition modules 260 may varyaccording to the type of signal that needs to be accepted and thussupports the different peripheral environments.

In the embodiment of FIGS. 4 and 5, the functions of reference signalsource 47, comparator 41, data latch 81, ramp generator 83,analog-to-digital converter 71 and digital-to-analog converter 73 of theembodiment of FIG. 3 are performed using modules that are part ofconfigurable control system 600 of FIG. 5. As shown in FIG. 4,input-cell 540 and signal acquisition module 260 of FIG. 5, properlyconfigured, serve to measure V(u) and express it in digital values.Synchronizing signal generator 250 and synchronization control module270 of configurable control system 600 are used to provide the waveformV(sync) of FIG. 4, which is substantially similar to the functionalityof V (rampd) and digital-to-analog converter 73 of FIG. 3. Acompensation circuit resistor Rcs 35 of FIG. 4 defines the offsetcurrent signal i(o) range. Signal acquisition module 260 can beconfigured to perform all control functions required to maintain alinear measurement of light sensor 13, within its entire wide-rangeoutput, at its output marked output data 201, which is practically theequivalent of Output V(rd) of FIG. 3.

The light intensity acquired by a daylight sensor between daytime andnight varies in order of magnitude 1000 and more. Using any of theembodiments described above, the available output marked V(r), V(rd) oroutput data 201 will enable linear measurement of the daylight sensorwith good sensitivity over the entire illumination range and without theuse of a logarithmic amplifier.

It will be appreciated where such analog light sensors 13 do notinherently generate sensor signals of the type that are describedherein, simple pre-conditioning circuits, known to those of skill in theart, can be used to pre-condition the sensor signal so that it iscompatible with the sensing and conditioning system 11 described herein.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   11 sensing and conditioning system-   13 analog light sensor-   15 photodiode-   21 operational amplifier-   23 inverse input of operational amplifier-   25 non-inverse input of operational amplifier-   27 operational amplifier output-   29 ground-   31 first amplifier feedback loop-   32 second amplifier feedback loop-   33 feedback resistor-   35 resistor-   41 comparator-   43 first comparator input-   45 second comparator input-   47 reference signal source circuit-   49 comparator output-   51 control circuit-   53 control circuit input-   55 control circuit output-   61 current source circuit-   71 analog-to-digital converter (ADC)-   73 digital-to-analog converter (DAC)-   81 data latch circuit-   83 ramp generator-   200 configurable digital unit-   201 output data-   210 signal input pins-   220 comparator output signal-   250 synchronization signal generator-   255 waveform vsync-   260 signal acquisition module-   270 synchronization control module-   275 sync data-   280 configurable output control logic module-   290 output reference data-   310 high-side switch control-   320 high-side switch-   330 low-side switch control-   340 low-side switch-   350 high-side output driver-   360 low-side output driver-   370 control logic module-   380 output pins-   522 first input-   540 input cell-   600 configurable control system-   i(d) sensor current signal-   i(f) feedback current signal-   i(o) offset current signal-   Vsync synchronization voltage-   V(cc) amplifier supply voltage-   V(r) control circuit output signal-   V(rd) digital control circuit output signal-   V(ramp) ramp output signal-   V(rampd) digital ramp output signal-   V(ref) reference signal-   V(refd) digital reference signal-   V(out) comparator output signal-   V(outd) digital comparator output signal-   V(u) amplifier output signal-   V(ud) digital output signal

1. A sensing and conditioning system comprising: an analog light sensor capable of converting a sensed input surface into an analog sensor current signal indicative of a range of, an amount of, an intensity of, or an exposure of the sensor to light; an operational amplifier having an inverse input connected to the analog light sensor, and a non-inverse input being connected to a ground, the amplifier providing an amplifier output signal at an amplifier output; an amplifier feedback loop having a resistor connected between the amplifier output and its inverse input to provide a feedback current signal at the amplifier inverse input; a comparator having a first comparator input connected to the output of the operational amplifier and a second comparator input connected to a reference circuit providing a reference signal, so that the comparator provides an output signal at a comparator output that reflects a comparison between the amplifier output signal and the reference signal; a control circuit having a control circuit input connected to the output of the comparator with said control circuit receiving the output signal from the comparator and, in response thereto, generating a control circuit output signal that has at least an integration component; and an offset source circuit having an offset input connected to the control circuit output and an offset output connected to the inverse input of the operational amplifier, and being adapted to generate an offset current signal at the offset output based upon the control circuit output signal, said offset current signal tending to minimize the difference between the light sensor signal and the feedback signal, so that said amplifier output signal will be controlled to be close enough to the reference signal to cause the control circuit output signal to be proportional to the light sensor signal.
 2. The sensing and conditioning system of claim 1, wherein the operational amplifier is a trans-impedance operational amplifier.
 3. The sensing and conditioning system of claim 1, wherein the analog light sensor generates an output signal that comprises a transducer that generates an analog output signal in proportion to the amount of light incident on a sensing surface.
 4. The sensing and conditioning system of claim 1, further comprising an analog-to-digital converter converting the an output signal at the output of the amplifier into an amplifier output signal in digital form, wherein the reference signal source provides the reference signal in digital form, and the comparator compares the amplifier output signal to the digital reference signal in digital form.
 5. The sensing and conditioning system of claim 4, further comprising a digital-to-analog converter adapted to receive a digital output signal from the control circuit and to provide a control circuit output signal in analog form.
 6. A sensing and conditioning system adapted for use with a light sensor that senses light and that provides a sensor signal that is proportional thereto, said sensor signal having a substantially linear wide-range of response to said input stimulus, the circuit comprising: an operational amplifier having an input connected to the light sensor to receive the sensor signal; a first feedback loop having a feedback resistor connected between an output of the operational amplifier and the input of the operational amplifier to which the sensor is connected to apply a first feedback signal to the input based upon an amplifier output signal provided at the amplifier output; a second feedback loop connected between the output of the operational amplifier and the input of the operational amplifier having: a source of a reference signal; a comparator adapted to compare the reference signal to the output signal from the amplifier and to generate a comparator output signal; and a control circuit adapted to generate a control circuit output signal that is based upon the comparator output signal and that drives an offset source to generate an offset signal to interact with the sensor signal and the first feedback signal at the input of the operational amplifier; said control circuit output signal having at least an integration component relative to the comparator output signal, said control circuit output signal causing said offset source to generate an offset signal that is applied to the input of the operational amplifier, so that the combined effect of the first and second feedback loops tends to keep the control circuit output signal proportional to the sensor signal and within a range of output signals that the amplifier is capable of generating.
 7. The sensing and conditioning system of claim 6, wherein the control circuit comprises an adaptive controller.
 8. A method for conditioning a signal from a sensor, said sensor generating an input current signal in response to the sensed light intensity, the method comprising the steps of: receiving the input current signal from the sensor; converting the input current signal into an amplified output voltage signal; comparing the amplified output voltage signal to a reference voltage signal; generating a comparator output signal based upon said comparing; and generating a control circuit output signal based upon the comparator output signal; said control circuit output signal being adapted to drive an offset source circuit to generate an offset current signal to interact with the input current signal and the first feedback signal; wherein said control circuit output signal having at least an integration component relative to the comparator output signal, said control circuit output signal causing said offset source to generate an offset current signal that is applied to the input of the amplifier, so that the combined effect of the first and second feedback loops tends to keep the control circuit output signal proportional to the input current signal from the sensor. 